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This is really going to be tough I guess.
As someone has already said, some 8-year old children can understand programming. There are two problems. One is the time limit of course. The other one is that lots of people just don't understand computers at all, and they can hardly be thought. This is true not only for adults, but for collage students too. I don't know whether this is determined at such an early age. If it is, some children will probably like your presentation very much, others won't like it at all. If it isn't, then your presentation might actually turn their interest towards computers.
Anyway, good luck for the presentation.
Update 2006-06-02: see also the later thread Teaching Children How to Program.
Update 2014-03-27: see the very similar Re: OT: Teaching Second Graders Programming.
In reply to Re: How to introduce 8 year olds to (Perl) programming?